Terahertz Radiation

Optical Pump THz Probe Spectroscopy THz is Far-Infrared radiation located between microwaves and infrared in the electromagnetic spectrum.

1 THz ~ 300 µm wavelength ~33 wave numbers ~ 4 meV

In THz Time Domain Spectroscopy, a ~1ps single-cycle THz pulse is propagated through a sample. The electric field of the THz pulse is sampled via the electrooptic effect in a second order nonlinear material. The field transmitted through the sample is compared to that transmitted through its substrate (or through free space for free standing samples) and through fourier analysis the frequency dependent index of refraction and absorption coefficients are extracted.

Above: The index of refraction of amorphous polycarbonate from 0-7THz.

In Optical-Pump THz-Probe Spectroscopy, a THz pulse is propagated through a photo-excited sample. The THz waveform transmitted through the excited sample is compared to that transmitted through the unexcited sample and through fourier analysis the full frequency dependent complex conductivity can be obtained. Modeling on this photo-induced conductivity yields the mobility and carrier density, also giving the photon-to-carrier yield. Control of the delay between the excitation (the "pump") and the THz pulse (the "probe") provides the time evolution of the photoexcited state.

Above: (left) Photoconductivity dynamics described by pump delay dependent changes in THz transmission through the photoexcited sample. (Left) The change in the transmitted THz waveform is recorded at various delays to allow for the extraction of the evolving frequency-dependent dielectric properties.

To extract the complex conductivity we use the following approximate analytic relationship following the work of Sundstrum

Advantages

- Non-contact low energy (meV) non-perturbing optical probe with sub-ps resolution

   - Coherent detection provides magnitude and phase of THz waveform, i.e. direct access to the complex dielectric properties of materials under study

   - THz radiation is sensitive to charge carriers as carrier scattering times are fs-ps placing the scattering (damping) rate in the THz regime

This technique has be used to examine carrier dynamics, Drude-like and deviations from the Drude model in popular semiconductors such as semi-insulating (SI-) GaAs

       Above: Excitation at 800nm vs 400nm. (Left) A band diagram of GaAs shows that the excess 1.49eV provided by the 400nm excitation allows carriers to scatter to higher valleys. (Right) The longer rise time of the 400nm photo-induced conductivity signal corresponds to the time required for carriers to return to the high mobility gamma valley.

       Above: The frequency-dependent photoconductivity of GaAs shows deviations from the simple Drude model.

       It can also be used to identify carrier trapping, as seen here in the comparison of the photo- conductivity of low-temperature grown (LT-) GaAs with SI-GaAs (below). The rapid decrease in THz absorption (below) in photoexcited LT-GaAs is due to carrier trapping by As clusters.

We have applied this technique to study carrier dynamics in RR-P3HT and ultrafast charge transfer in the bulk heterojunction P3HT/PCBM 1:1 excited with either 400 nm (3.1 eV) and 800 nm (1.44 eV) light, the results of which are published in J Phys Chem C. Above: (Left) Inclusion of a liquid He cryostat allowed for temperature dependent studies (Right) At low temperature there is an increase in the photoconductivity (mobility) of P3HT due to decreased torsoinal disorder which increases the effective conjugation length. The faster recombination dynamics are associated with inhibited interchain hopping at low temperature.

Above: Though there is only very weak absorption at 800 nm, both energy excitations yield similar dynamics on subpicosecond time scales.

Above: Ultrafast electron transfer from P3HT to PCBM is marked by an increase in the long lived photoconductive state in the bulk heterojunction compared to neat films of P3HT.

The extracted complex frequency-dependent conductivty of P3HT exibits characteristics of strong (Anderson) carrier localization and inhibited long range trasport due to disorder (below). Fitting of the conductivity to the Drude-Smith model provides photon-to-carrier yields of < 1.5% and a hole mobility of ~35 cm^2/Vs. The THz regime mobility agrees well with published DFT calculations of the predicted intrinsic hole mobility in well ordered P3HT.

Above: (Left) The complex conductivity of P3HT measured from 200 GHz - 2.1 THz extracted at 1, 1.5 and 6.2 ps after photoexcitation. The solid lines are fits to the Drude-Smith model. (Right) The mobility in perfectly ordered P3HT far exceeds that typically measured in devices due to the effect of disorder and defects.

The high frequency THz radiation probes carriers over short (nanoscale) distances, which corresponds to highly ordered domains in this system. Through this experiment we are able to experimentally determine the upper limit of the carrier mobility that would be seen in a device with perfect ordering, instead of the disorder limited mobility typically seen in devices.

See Wikipedia Terahertz Radiation See Wikipedia Terahertz time-domain spectroscopy